The key difference between polycyclic and polynuclear aromatic hydrocarbons is that the term polycyclic aromatic hydrocarbon describes the presence of two or more cyclic structures fused to each other whereas the term polynuclear aromatic hydrocarbon describes the presence of more than one atom.
The terms polycyclic aromatic hydrocarbons and polynuclear aromatic hydrocarbons refer to the same group of organic compounds that contains several cyclic structures of carbon and hydrogen are fused with each other forming a large organic molecule. However, the difference between polycyclic and polynuclear aromatic hydrocarbons lies on the description given by each term; polycyclic refers to “many cycles” while polynuclear refers to “many atoms”.
CONTENTS
1. Overview and Key Difference
2. What are Polycyclic Aromatic Hydrocarbons
3. What are Polynuclear Aromatic Hydrocarbons
4. Side by Side Comparison – Polycyclic vs Polynuclear Aromatic Hydrocarbons in Tabular Form
5. Summary
What are Polycyclic Aromatic Hydrocarbons?
Polycyclic aromatic hydrocarbons are a group of organic compounds which has large organic molecules having several cyclic structures fused with each other. As its name implies, polycyclic aromatic hydrocarbons are hydrocarbon compounds (these molecules have only carbon and hydrogen atoms) which are aromatic (contains planar, unsaturated rings of atoms with increased stability due to the delocalization of pi electrons) and contain fused aromatic ring structures. The denotation for these compounds is PAH.
PAHs are uncharged compounds, and they are mostly nonpolar as well. We can find these compounds in coal and tar deposits. The thermal decomposition of organic matter produces polycyclic aromatic hydrocarbons. The simplest PAH is naphthalene. Although benzene is an aromatic cyclic structure, we cannot consider it as a polycyclic hydrocarbon because it has only one cyclic structure (a PAH contains more than one cyclic structure). Usually, these compounds do not contain heteroatoms (atoms other than carbon and hydrogen). Moreover, they usually do not carry substituents.
Figure 01: Different Polycyclic Aromatic Hydrocarbon Compounds
These are nonpolar and lipophilic compounds. Since they are organic and aromatic, PAHs are insoluble in water. However, some compounds can be seen as contaminants in drinking water. The large members of this class are insoluble, even in organic solvents and lipid solutions. Moreover, these compounds are usually colourless.
What are Polynuclear Aromatic Hydrocarbons?
Polynuclear aromatic hydrocarbons are aromatic organic compounds which contain more than one atom. This class of compounds contains polycyclic aromatic compounds and other aromatic compounds containing more than one atom. The term polynuclear refers to many+nuclei (atomic nuclei).
Figure 02: Benzene Structure
However, we usually use this term to name polycyclic aromatic hydrocarbon compounds. But, this category also contains benzene because it is an aromatic compound having 18 atoms.
What is the Difference Between Polycyclic and Polynuclear Aromatic Hydrocarbons?
Although the terms polycyclic aromatic hydrocarbon and polynuclear aromatic hydrocarbon refers to the same set of compounds, these two terms define them differently. The key difference between polycyclic and polynuclear aromatic hydrocarbons is that the term polycyclic aromatic hydrocarbons describe the presence of two or more cyclic structures fused to each other, whereas the term polynuclear aromatic hydrocarbons describe the presence of more than one atom.
Moreover, the benzene molecule is not included in the class of polycyclic aromatic hydrocarbons because it has only a single cyclic structure. However, it is included in the group of polynuclear aromatic hydrocarbons. Therefore, this too is a significant difference between polycyclic and polynuclear aromatic hydrocarbons.
Summary – Polycyclic vs Polynuclear Aromatic Hydrocarbons
Although the terms polycyclic aromatic hydrocarbon and polynuclear aromatic hydrocarbon refers to the same set of compounds, these two terms define them differently. The key difference between polycyclic and polynuclear aromatic hydrocarbons is that the term polycyclic aromatic hydrocarbons describes the presence of two or more cyclic structures fused to each other whereas the term polynuclear aromatic hydrocarbons describes the presence of more than one atom.
Reference:
1. Helmenstine, Anne Marie. “What Is a Polynuclear Aromatic Hydrocarbon?” ThoughtCo, Nov. 25, 2019, Available here.
Image Courtesy:
1. “Chrysene Clar rule” By Dissolution – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Benzene-aromaticity-3D-balls” By Benjah-bmm27 – Own work (Public Domain) via Commons Wikimedia